In general, the vehicular transmission is assembled in an automatic transmission system which is of a step-variable transmission type or of a continuously variable transmission type. The automatic transmission system comprises an oil cooler having oil pass therethrough to lubricate and cool the transmission. For selectively passing the oil through the oil cooler, a thermo-valve is used to change oil passageways in response to the temperature of the oil.
As one of the thermo-valves of this kind, there has so far been known a thermo-valve of a wax accommodating type to selectively open and close the oil passageways held in communication with first and second heat exchangers. The first heat exchanger functions to exchange the heat of the oil for lubricating and cooling the transmission with the heat of cooling water for cooling an engine after the cooling water has passed through a heater core. The second heat exchanger is an oil cooler of an air-cooling type and disposed between the automatic transmission and the first heat exchanger to exchange the heat of the oil returning to the automatic transmission from the first heat exchanger with the heat of air introduced from the front of an automotive vehicle, so that the second heat exchanger can cool the oil returning to the automatic transmission from the first heat exchanger with by opening the oil passageway to the second heat exchanger with the thermo-valve when the temperature of the oil becomes higher than the preliminarily set temperature.
The thermo-valve of the wax accommodating type comprises a valve for opening and closing a bifurcated oil passageway allowing the oil to be introduced into the second heat exchanger, a valve supporting member in a bottomed cylindrical shape for supporting the valve, and a supporting shaft in a piston shape for movably supporting the valve supporting member. The valve supporting member has wax accommodated therein with the wax being expandable and contractible in response to the temperature of the wax. The thermo-valve further comprises a compression spring for resiliently urging the valve so that the valve can be opened and closed by means of the temperature sensitive wax and the compression spring. There has so far been encountered such a problem that the oil is difficult to be filled into the oil circuit of the second heat exchanger, which can be opened for the oil maintained at a high temperature, in the state in which the oil in the oil circuit is maintained at a low temperature at the stage of manufacturing vehicles. In order to overcome such a problem, there has so far been proposed one of the thermo-valves which comprises an engagement pin in addition to the valve, the supporting member, the supporting shaft, and the compression spring constructed in the previously mentioned thermo-valve. The engagement pin is designed to secure the supporting shaft to a valve housing or case in a forced opened state in which the valve supporting member is moved to have the valve opened against the compression spring. After the oil is filled into the oil circuit of the second heat exchanger, the engagement pin is removed from the supporting shaft and the valve housing before the supporting shaft and the valve supporting member are returned to their respective original positions (see for example Patent Literature 1).